U.S. patent number 6,922,171 [Application Number 09/792,354] was granted by the patent office on 2005-07-26 for planar antenna structure.
This patent grant is currently assigned to Filtronic Lk Oy. Invention is credited to Petteri Annamaa, Jyrki Mikkola.
United States Patent |
6,922,171 |
Annamaa , et al. |
July 26, 2005 |
Planar antenna structure
Abstract
The invention relates to planar antennas the structural
components of which include a parasitic element. The antenna
structure comprises a PIFA-type structure (230, 210, 202) to be
placed inside the covers of a mobile station. The PIFA is fed
parasitically e.g. through a conductive strip (240) placed on the
same insulating board. The feed conductor (203) of the whole
antenna structure is in galvanic contact with this feed element; a
short-circuit point the feed element doesn't have. The feed element
(240) also serves as an auxiliary radiator. The resonance
frequencies of the antenna elements or their parts are arranged
according to need so as to overlap, to be close to each other or to
be relatively wide apart. The structure may also comprise a whip
element in connection with the feed element. According to the
invention, a relatively simple structure provides a reliable dual
resonance and, hence, a relatively wideband antenna when the
resonances are close to each other. Moreover, no polarization
rotation takes place in the antenna radiation inside the frequency
band realized through the dual resonance.
Inventors: |
Annamaa; Petteri (Oulu,
FI), Mikkola; Jyrki (Kempele, FI) |
Assignee: |
Filtronic Lk Oy (Kempele,
FI)
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Family
ID: |
8557726 |
Appl.
No.: |
09/792,354 |
Filed: |
February 23, 2001 |
Foreign Application Priority Data
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Feb 24, 2000 [FI] |
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20000437 |
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Current U.S.
Class: |
343/700MS;
343/702 |
Current CPC
Class: |
H01Q
1/243 (20130101); H01Q 9/0421 (20130101); H01Q
5/392 (20150115) |
Current International
Class: |
H01Q
9/04 (20060101); H01Q 5/00 (20060101); H01Q
1/24 (20060101); H01Q 001/24 () |
Field of
Search: |
;343/700MS,829,846,702 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 332 139 |
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Sep 1989 |
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EP |
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0 790663 |
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Aug 1997 |
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EP |
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0 831547 |
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Apr 1998 |
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EP |
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0 831548 |
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Apr 1998 |
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EP |
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1 024 552 |
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Aug 2000 |
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EP |
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1 139 490 |
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Oct 2001 |
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EP |
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WO 98/38694 |
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Sep 1998 |
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WO |
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WO-99/03166 |
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Jan 1999 |
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WO |
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WO-01/24314 |
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Apr 2001 |
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WO |
|
Other References
European Search Report; EP 01 66 0016; Documents considered to be
relevant. .
"Improved analysis method for broadband rectangular microstrip
antenna geometry using E-plane gap coupling," Cho, YK, Electronics
Letters, Oct. 28, 1993, vol. 29, No. 22, pp. 1907-1909..
|
Primary Examiner: Phan; Tho
Attorney, Agent or Firm: Darby & Darby
Claims
What is claimed is:
1. An antenna structure comprising a ground plane, one and only one
planar feed element, one and only one planar parasitic element
(230), one and only one feed conductor (203) and one and only one
short-circuit conductor, the feed conductor being coupled to the
feed element (240) and the short-circuit conductor being connected
to the parasitic element, which, being larger in surface area than
the planar feed element, is a main radiating element, and said feed
element, which is not connected to the short-circuit conductor,
being electromagnetically coupled to said parasitic element.
2. A structure according to claim 1, characterized in that said
feed element is arranged to resonate at substantially the same
frequency as said parasitic element.
3. A structure according to claim 1, characterized in that said
parasitic element (230) and said feed element (240) are separate
conductive regions on a surface of a dielectric plate (208).
4. A structure according to claim 1, characterized in that said
parasitic element (330) and said feed element (340) are separate
self-supporting conductive bodies.
5. A structure according to claim 1, characterized in that said
parasitic element viewed from said short-circuit point (S), is
divided into two branches having certain resonance frequencies.
6. A radio apparatus (MS) comprising an antenna (700) having a
ground plane, one and only one planar feed element, one and only
one planar parasitic element, one and only one feed conductor and
one and only one short-circuit conductor, the feed conductor being
coupled to the feed element and the short-circuit conductor being
connected to the parasitic element, which, being larger in surface
area than the planar feed element, is a main radiating element, and
said feed element, which is not connected to the short circuit
conductor, being electromagnetically coupled to said parasitic
element, which parasitic element is short-circuited at a single
point to the ground plane.
7. An antenna structure comprising a ground plane, planar feed
element and a planar parasitic element, characterized in that said
feed element (240) is coupled to a feed conductor (203) of the
antenna structure and electromagnetically coupled to said parasitic
element (230) which is short-circuited at a single point (S) to the
ground plane, wherein the antenna structure additionally comprises
a whip element which, when pulled out, is in galvanic contact with
said feed element.
Description
FIELD OF INVENTION
The invention relates to planar antennas the structural parts of
which include a parasitic element. The antenna finds particular
utility in mobile stations which require a relatively wide band or
which are to be used in two or more frequency bands.
BACKGROUND
In portable radio apparatuses, especially in mobile stations, the
antenna requirements have become more severe. As the devices
continue to shrink in size, the antenna naturally has to be small;
preferably it is placed inside the covers of the apparatus. On the
other hand, together with the introduction of new frequencies there
has been a growing demand for mobile stations in which the antenna
must function in two or more frequency bands. In addition, in
dual-band antennas the upper operating band at least should be
relatively wide, especially if the device in question is to be used
in more than one system utilizing the 1.7 to 2-GHz range.
Antenna requirements may be met through various structural
solutions. The solution according to the present invention is based
on the application of a parasitic element in planar antennas.
Several such structures are known in the art. Typically they
comprise a printed circuit board with a ground plane on one surface
and a conductive region connected to an antenna feed line and at
least one parasitic conductive region on the other surface. Such a
structure is shown in FIGS. 1a,b. FIG. 1a shows a top view of an
antenna 100, and FIG. 1b shows a side view of a cross section of
the same antenna. The structure comprises a dielectric plate 108.
On the upper surface of the plate 108 there are conductive regions
120 and 130 which function as radiating elements. On the lower
surface of the plate 108 there is a conductive region 110 which
covers the whole surface and functions as a ground plane. The first
radiating element 120 is connected at a point F through a feed
conductor 102 to a source feeding the antenna. In addition, the
element 120 is short-circuited to ground at a point S through
conductor 103 so as to improve the electrical characteristics, such
as impedance matching, of the antenna. The resulting structure is
called a planar inverted F antenna (PIFA). The second radiating
element 130 is parasitic, i.e. there is only an electromagnetic
coupling between it and the first element 120. It, too, may have a
short-circuit point. The purpose of the parasitic element is to
further improve the electrical characteristics, such as bandwidth
or radiation pattern, of the antenna.
One drawback of the above-described antennas according to the prior
art is that their bandwidth is not always large enough for modern
communications devices.
Radiating elements may be designed such that the bandwidth is
increased through two adjacent resonance frequencies, but then the
disadvantage of the structure is that the structure is relatively
complex as regards ensuring reliable operation. An additional
disadvantage of an element, which has two adjacent resonances, is
that the polarization of its radiation rotates inside the band.
Moreover, it is a disadvantage of the structures described above
that they are sensitive to the effect of the user's hand, for
example. If a finger, for instance, is placed over the radiating
element of a PIFA on the outer cover of the apparatus, the
operation of the PIFA will be impaired.
BRIEF SUMMARY OF THE INVENTION
An object of the invention is to reduce the above-mentioned
disadvantages associated with the prior art. The antenna structure
according to the invention is characterized by what is specified in
the independent claim 1. Advantageous embodiments of the invention
are specified in the dependent claims.
The basic idea of the invention is as follows: The antenna
structure comprises a PIFA-type element to be placed inside the
covers of a mobile station. The PIFA is fed parasitically e.g.
through a conductive strip on the same insulating board. The feed
conductor of the whole antenna structure is connected galvanically
to this feed element; a short-circuit point the feed element
doesn't have. At the same time the feed element serves as an
auxiliary radiator. The ground plane of the antenna is a separate
element located relatively far away from the radiating elements.
The resonance frequencies of the antenna elements or their parts
are arranged according to need so as to overlap, to be close to
each other or to be relatively wide apart. The structure may also
comprise a whip element in connection with the feed element.
An advantage of the invention is that with a relatively simple
structure a reliable dual resonance can be achieved and, hence, a
relatively wideband antenna when the resonances are close to each
other. Another advantage of the invention is that a relatively
large gain can be achieved for the antenna by utilizing overlapping
resonances. A further advantage of the invention is that the
antenna can be easily made a dual-band antenna by arranging the
resonance frequencies such that they fall into the frequency bands
used by the desired systems. A still further advantage of the
invention is that no polarization rotation will take place in the
antenna radiation inside the frequency band realized through the
dual resonance. A yet further advantage of the invention is that
the manufacturing costs of the structure are relatively low as it
is simple and suitable for series production.
The invention is described in detail in the following. The
description refers to the accompanying drawings, in which
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B show an example of an antenna structure according
to the prior art,
FIG. 2 shows an example of an antenna structure according to the
invention,
FIG. 3 shows another example of an antenna structure according to
the invention,
FIGS. 4A, 4B, 4C and 4D show other examples of antenna element
design,
FIG. 5 shows an antenna according to the invention with an
additional whip element,
FIG. 6 shows an example of the frequency characteristics of an
antenna according to the invention, and
FIG. 7 shows an example of a mobile station equipped with an
antenna according to the invention.
DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT OF THE
INVENTION
FIG. 1 was already discussed in conjunction with the description of
the prior art.
FIG. 2 shows an example of an antenna structure according to the
invention. In this example the antenna 200 comprises a ground plane
210 and a parallely positioned dielectric plate 208, attached to
the ground plane through insulating pieces such as 205. On the
outer surface, as viewed from the ground plane, of the dielectric
plate 208 there are two separate planar conductive regions: a
parasitic element 230 and feed element 240. On the
ground-plane-side surface of the dielectric plate 208 there are no
conductive regions. The parasitic element is short-circuited at a
point S to the ground plane through conductor 202. The radiating
parasitic element 230, short-circuit conductor 202 and ground plane
thus constitute the PIFA-part of the antenna. The feed conductor
203 of the whole antenna structure is in galvanic contact with the
feed element 240 at a point F. The feed element has two functions.
It, too, serves as a radiating element and, on the other hand, it
transfers energy through an electromagnetic coupling to the field
of the parasitic element. Antenna characteristics are naturally
dependent on the relative positions of the elements: the wider
apart the elements, the smaller the bandwidth of a single-band
antenna and, correspondingly, the greater the Q value.
In the example of FIG. 2 the parasitic element has a slot 235 which
divides the element, viewed from the short-circuit point S, into
two branches the lengths of which are not equal. The PIFA thus has
got two natural frequencies. In the example depicted the feed
element has a slot 245 which is used to give a desired length for
the feed element, viewed from the feed point F. The frequency
characteristics of the antenna depend, in addition to the length
and mutual distance of the facing edges of the elements, on the
resonance frequencies of the elements and on their distance from
the ground plane. Each resonance frequency depends on the length of
the element or its branch. With the structure of FIG. 2 it is
possible to arrange the dimensions of the elements such that the
resonance frequency of the longer branch of the parasitic element
230 falls into the frequency band of the GSM 900 system (Global
System for Mobile telecommunications), for example, and the
resonance frequencies of the shorter branch of the parasitic
element and feed element fall into the frequency band of the GSM
1800 system. By taking the latter two resonance frequencies further
apart from each other the corresponding frequency band gets wider
until it is split into two separate frequency bands. It is
substantial in the invention that the parasitic element is
short-circuited but the feed element is not. Using these ways to
produce adjacent resonance frequencies one can achieve relatively
large bandwidths more simply than in the prior art. Another
significant fact is that no polarization rotation occurs in the
antenna radiation inside the frequency band realized by means of
the dual resonance, unlike in corresponding structures according to
the prior art.
FIG. 3 shows another example of an arrangement according to the
invention. It comprises a planar feed element 340, planar parasitic
element 330 and, behind those, a ground plane 310. In this example,
too, the parasitic element includes a slot which divides the plane,
viewed from the short-circuit point S, into two unequally long
branches so as to produce a dual-band antenna. The feed conductor
of the whole antenna structure is at point F in galvanic contact
with the feed element 340. The difference from the structure of
FIG. 2 is that now the parasitic element and feed element are not
conductive regions on the surface of a dielectric plate but
discrete and rigid conductive bodies.
FIGS. 4a-d show additional examples of antenna element design
according to the invention. In each of the FIGS. 4a, 4b and 4c the
parasitic element 431; 432; 433 is a dual-frequency element and the
feed element 441; 442; 443 has dimensions such that its resonance
frequency comes relatively close to the upper resonance frequency
of the parasitic element. The ground plane, not shown, is at a
distance that equals a little less than half of the shorter side of
the rectangle formed by the radiating elements. These structures
are suitable for communications devices designed to function in the
GSM 900 and GSM 1800 systems, for example. In FIG. 4d the parasitic
element 434 has got two branches as well. Now, however, the
structural dimensions of both said parasitic element and the feed
element are chosen such that all resonance frequencies of the
antenna fall into the frequency band 1900 to 2170 MHz allocated to
the Universal Mobile Telecommunication System (UMTS), for
example.
FIG. 5 shows an embodiment in which an antenna according to the
invention is supplemented with a whip element. The basic structure
is similar to that of FIG. 2. In addition, there is a whip element
550, shown in its extended position. In this example it is thus in
galvanic contact with the feed element 540 through a connection
piece 551. The mechanism that presses the connection piece against
the feed element is not shown. The whip is coupled to that end of
the feed element which is opposite to the feed point F. By means of
the feed element can be arranged the electrical length of the whip
greater than its physical length. The whip is made to resonate e.g.
in the upper frequency band of the PIFA part. When the whip is in
its pushed-in position, there is no significant coupling between it
and the other parts of the antenna structure.
FIG. 6 shows an example of the frequency characteristics of an
antenna according to the invention. It shows a curve 61 for the
reflection coefficient S11 as a function of frequency. The antenna
in question is designed for UMTS devices. The curve shows that in
the UMTS frequency band the reflection coefficient of the antenna
varies between -8 . . . -15 dB, which indicates relatively good
matching and radiation power.
FIG. 7 shows a mobile station MS. It includes an antenna structure
700 according to the invention, located completely within the
covers of the mobile station.
Above it was described some antenna structures according to the
invention. The invention does not restrict the antenna element
designs to those described above. Nor does the invention restrict
in any way the manufacturing method of the antenna or the materials
used therein. The inventional idea may be applied in different ways
within the scope defined by the independent claim 1.
* * * * *